1. Field of the Invention
The present invention relates to a solid state image pick-up device for use in an information processing apparatus such as a copying machine or facsimile terminal equipment, and more particularly to a solid state image pick-up device with multiple pixels arranged and having a photoelectric conversion unit in which a signal corresponding to electric charge stored in the control electrode area by light illumination is output from one of the second main electrode areas.
2. Related Background Art
In recent years, solid state image pick-up devices of the CCD type in which a CCD is used in the scanning circuit have been typically well known. Also, because the output of a photoelectrically converted signal may decrease with higher definition of the photoelectric conversion elements, photoelectric conversion elements of the amplification type in which a photoelectrically converted signal can be amplified for output have gained popularity. Such amplification type photoelectric conversion elements include a bipolar transistor type, a field effect transistor type and an electrostatic induction transistor type. Among them, photoelectric conversion elements of the bipolar transistor type include a normal photo-transistor, as well as a photo-transistor in which the emitter is connected to a capacitive load as disclosed in U.S. Pat. No. 4,791,469 issued to the inventors Ohmi and Tanaka. Such a photoelectric conversion element of the amplification type with a bipolar transistor will be exemplified in the following.
FIG. 15 is a schematic plan view showing a linear solid state image pick-up device using the above-mentioned bipolar sensor. In FIG. 15, 1 is a part of a semiconductor substrate which is a collector region of the bipolar type sensor. In addition, 2 and 3 are a base region and an emitter region of the bipolar type sensor, respectively, and 4 is a read-out circuit system which outputs a signal from each pixel sequentially. U (as indicated in the broken line) shows a light sensitive pixel (photoelectric conversion element) to provide a bipolar type sensor. Plural light sensitive pixels of the same structure are arranged in a linear format. It is to be noted herein that each pixel is shielded against light except for a light receiving face having an open area (x times y), wherein if these light receiving faces are equal in size (x times y), each pixel theoretically will have the same light sensitivity.
FIG. 16 is a schematic plan view showing a two-dimensional solid state image pick-up device using the above-mentioned bipolar type sensors. In FIG. 16, 11 is a part of the semiconductor substrate which is a collector region of the bipolar type sensor. Also, 12 and 13 are a base region and an emitter region of the bipolar type sensor for each pixel, respectively, and 14 is a read-out circuit system which outputs a signal from each pixel sequentially. U' (as indicated in the-broken line) shows a light sensitive pixel (photoelectric conversion element) providing a bipolar type sensor. Plural light sensitive pixels of the same structure are arranged in a two-dimensional format of m.times.n pixels. It is to be noted herein that if the light receiving faces of the pixels have light receiving areas or open areas or open areas of equal size, each pixel theoretically will have the same light sensitivity.
For the purpose of reading a monochromatic image, the constitutions of FIGS. 15 and 16 are most suitable.
However, since light signals from solar rays or lamp rays have spectral characteristics over wide wavelength regions, if a plurality of light signals which lie in mutually different wavelength regions are photoelectrically converted by different pixels, there may occur differences in sensitivity.
To facilitate understanding of this technical problem, the reading of a color image will be described below.
When reading a color image with a solid state image pick-up device having red light signal reading pixels (R pixels), green light signal reading pixels (G pixels) and blue light signal reading pixels (B pixels), it is common to dispose one color filter on each pixel in accordance with a certain rule. In this case, because the spectral transmission characteristics of color filters are different for each color, the sensitivity of a pixel having a color filter disposed is a value determined by the characteristic of the color filter, which may be different for each color. For example, when using red R, green G and blue B primary color filters, the sensitivity often has the relation that red sensitivity SR&gt;green sensitivity SG&gt;blue sensitivity SB.
An image read by such a color solid state image pick-up device is converted into an electric signal for each color, and output to a signal processing circuit at a later stage. In this case, because the sensitivity is different for each color owing to the reason previously mentioned, it is necessary to apply a different gain to each color output signal prior to its input into the signal processing circuit at the later stage, for example, so that the signal level of each color in reading with white uniform light may be equal. Such processing is referred to as white balance processing.
This white balance processing necessitates excess hardware or excess control at the system level, leading to an increase in cost. As one means for resolving this problem, it is contemplated that the sensitivity of each color is adjusted by varying the open area of the pixel for each color filter, but there is a risk that a different aperture characteristic for each color may be caused.